This application is designed as a supplement to the SPAF III clinical trial and requests support for a study of TEE of patients participating in SPAF III. The specific aim of the study is to determine the value of TEE in identification of patients with nonvalvular AF who require life- long anticoagulation to prevent an ischemic stroke.Specifically, this study proposes to determine whether spontaneous echo-contrast is a predictor of thromboembolism in prospectively defined cohorts of high- risk and low-risk AF patients, to compare the prevalence and characteristics of spontaneous echocardiographic contrast in patients who respond adequately to aspirin prophylaxis to findings in patients who developed thromboembolic events during aspirin therapy, and to evaluate transesophageal echocardiographic variables in comparison with clinical and transthoracic echocardiographic variables as the basis for selection of antithrombotic therapy for patients with AF. The investigation will be carried out in 15 centers in adults with electrocardiographic evidence of AF. TEE will be performed in those granting additional consent. Then, the study will assess aspirin in so- called "low risk" group of patients, and adjusted-dose warfarin vs. small dose warfarin plus aspirin as methods of anti-coagulation in the high risk group. The investigators hypothesize that spontaneous left atrial contrast on TEE identifies patients for whom aspirin alone is inadequate, and that adjusted dose warfarin is superior to the low-dose warfarin plus aspirin in high risk patients with spontaneous contrast.